MFJ-269D Instruction Manual LF/HF/VHF/UHF SWR Analyzer
11
SWR is always your best predictor of antenna performance.
4. Tuning and Matching:
Unlike simple wire dipoles, many antennas such as Yagis and verticals are
adjustable for both
resonant frequency
and
impedance match
. Begin by setting these antennas for the
element length prescribed in the instruction sheet. Then, adjust the matching network for minimum SWR.
The two adjustments are separate, but often interact. Be prepared to alternately readjust both the element
length and the matching network to achieve minimum SWR on your frequency of interest.
5. Adding and Removing Feedline:
You should be able to add or remove lengths of feedline (or to
measure SWR at any point along your feedline) without observing a significant change in SWR. It is
normal to see SWR
drop slightly
as cable is added, or see it
increase slightly
as cable is removed because
of a change in resistive loss. However, (a.) if your SWR measurements
change a lot
with relatively small
changes in cable length, or (b.) SWR changes as the cable is moved around, or (c.) SWR changes when
the coax shield is grounded at some point part way between the antenna and the radio, look for a feed
problem! Here are some possibilities to check:
6. Common Mode Current
:
Your coax may be carrying
Common-Mode Current
on its outer shield and
radiating RF. To eliminate this problem, install a Guanella current balun at the feedpoint. It will isolate
the outer coax shield from the radiating portion of the antenna, stabilize your SWR, reduce receiver
noise, and suppress "RF in the shack". Installing a balun is good engineering practice and always worth
doing!
7. Defective Cable:
Your coax may not really be 50 ohms. Kinks, water ingress, oxidation, corrosion,
bad connectors, improper manufacturing, or even mislabeling may be the cause. Check SWR with a
dummy load installed at the far end of the cable. If the SWR is elevated or the
Impedance (Z)
fluctuates
very much as you tune the analyzer's VFO, suspect a defective cable.
8. Excessive Transmission Line Loss:
Your cable may exhibit unusually high loss because of damage
or contamination. Or, it may simply have too much normal attenuation for the frequency range where
you're using it (especially true at VHF and UHF). To measure loss, unterminate the cable at its far end
and use the analyzer's
Coax Loss
mode to check it.
9. Reactance Sign:
The MFJ-269D measures the antenna's reactance
(X)
and mathematically converts it
to a value. Unfortunately, the analyzer's processor can't determine if the reactance it measures is actually
inductive (
+jX
) or capacitive (
-jX
). However, you can often determine the reactance sign by installing a
small-value of capacitance across the antenna feedpoint. If the reactance
increases
, it is likely
capacitive
because the two are the same sign and add. If the reactance reading
decreases
, it is likely
inductive
because the reactance signs are opposite and subract. Note that the reactance of the added capacitor must
be quite small at the test frequency to avoid potential ambiguity.
4.3
Coax Loss (Function-2)
Bring up the analyzer's coax loss mode by stepping the
Mode
switch to the
Coax Loss
identification
screen. The top line of the working screen displays
Frequency
in MHz and the lower line shows
Coax
Loss
in
dB
. Note that the
Impedance
meter is disabled in this mode.
Coax Loss
was designed to measure
losses in 50-ohm cables, but also effectively measures differential-mode loss in many types of 50-ohm
transmission-line transformers, choke baluns, and 50 ohm attenuator pads.
CAUTION:
Only measure transformers or attenuators and coaxial cables that are 50-ohm devices. Also,
when making your measurement, confirm that the opposite end of the DUT (device under test) has an
open circuit, short circuit, or a purely reactive termination. Any resistive component added at the far-end
termination point will make attenuation (loss) appear worse than it actually is.
To measure loss: